Quote: Thanks - do you use a motor to stir the fluid to avoid convection
That's a point, haven't noticed it with the smaller prism, now I'll be looking.
-------------------- Jc
ATM 10" F6.1, 1/25th wave spec (max wavefront error +/- 1/12.6 in zone 4 of 6, sodium light )
6" F7 spec
127mm F9.4 Refractor
10 x 50 bin
ETX80 (finder)
Canon 20D
and a curious mind